Not Applicable.
Not Applicable.
1. Field of the Invention
The present invention relates to a magnetic thin film for a magnetic recording medium, and more particularly, to Ga3Pt5 structured underlayers for use with a cobalt or cobalt based magnetic layer.
2. Description of the Invention Background
In recent years there has been an ever-increasing demand for computers with greater data storage capacity. This demand has been met by the development of computer discs, both flexible and rigid, that contain magnetic recording media with a greater magnetic recording density. Data on the discs is stored in circular tracks and divided into segments within the tracks. Disc drives typically employ one or more discs rotated on a central axis. A magnetic head is positioned over the disc surface to either access or add to the stored information. The heads for disc drives are mounted on a movable arm that carries the head in very close proximity to the disc over the various tracks and segments. The structure of disc drives is well known.
Presently, the dominant type of disk is a thin-film disk comprised of a multilayer structure that includes a substrate at the base covered by an underlayer structure, a magnetic layer structure and optionally, an overlayer at the top. The overlayer is commonly coated with an overcoat and an organic lubricant. Sometimes an intermediate layer will be placed between the underlayer structure and the magnetic layer and in addition sometimes a seed layer will be placed between the substrate and the underlayer structure. Data, in the form of magnetic bits, is stored on the magnetic layer. The magnetic layer is typically comprised of cobalt or a cobalt based alloy with a hexagonal closed packed (xe2x80x9cHCPxe2x80x9d) structure, such as CoCrTa, CoCrPt, CoCrPtB, CoCrPtTa, and CoNiCrPt.
The microstructure of the magnetic layer is critical to achieving a high magnetic recording density. For thin film longitudinal magnetic recording media, the desired crystalline structure of the Co or Co alloy is HCP with uniaxial crystalline anisotropy and a magnetization easy direction along the c-axis, which is in the plane of the film. The better the in-plane c-axis crystallographic texture, the more suitable the Co alloy thin film is for use in longitudinal recording.
To achieve a high magnetic recording density, the magnetic layer must consist of small and isolated grains that reduce media noise. In addition, a key to increasing media recording density is to reduce transition length, which can be achieved by increasing media coercivity. High coercivity is achieved by obtaining Co grains of such crystalline perfection that the Co magneto-crystalline anisotropy is maximized and not compromised by lattice strains and defects. It is well known that, by obtaining a good Co crystallographic texture, the alignment of the Co easy axis in the film plane increases the coercivity of the media. There are two crystallographic textures that align the Co easy axis in the thin film plane, Co (1120) and Co (1010).
The desired microstructure of the magnetic layer can be achieved by manipulating the deposition process, by grooving the substrate surface, or, most commonly, by using an underlayer. It is well known that an underlayer can be used to control the texture and grain size of the magnetic layer. Various materials have been employed for use as an underlayer. In particular, for longitudinal media, NiAl, NiP, Cr and Cr alloys containing such elements as Mn, Ru, Ti, W, Mo or V have been used, although, among these materials, pure Cr and Cr alloys have been the most widely used. A Cr underlayer develops a Cr (002) texture when deposited at elevated temperatures, e.g., about 150 or 200xc2x0 C., which enables epitaxial growth of the Co (1120) textured thin film.
U.S. Pat. No. 5,693,426 to Lee et al., teaches the use of a material having a B2-ordered crystalline structure, and a combined layer structure of a B2 followed by a Cr or Cr alloy, as an underlayer structure. These B2 materials include NiAl, AlCo, FeAl, FeTi, CoFe, CoTi, CoHf, CoZr, NiTi, CuBe, CuZn, AlMn, AlRe, AgMg, and Al2FeMn2. The most preferable material is NiAl. The NiAl underlayer is preferable to the Cr underlayer because the unique NiAl (112) texture can be used to induce the uni-crystal Co layer of the Co (1010) texture. In addition, the NiAl underlayer induces smaller Co grains and a xe2x80x9ctighterxe2x80x9d grain size distribution than can be achieved with the Cr underlayer. Both of these factors are essential to reduce media noise.
Notwithstanding the benefits achieved by the use of materials having a B2-ordered crystalline structure as an underlayer, and NiAl in particular, the NiAl (112) plane is not the lowest surface-energy plane. There is a need for recording media having even greater storage density and improved texture of NiAl.
The present invention provides an improved recording media. The recording media of the invention may be used for incorporation in a disc drive having a rotatable disc for operation in conjunction with magnetic transducing heads for the recording and reading of magnetic data. It may also be used with storage devices such as flexible magnetic discs or tapes using known flexible substrates or x-y addressable storage systems that might use ridge substrates. The novel recording media comprises a flexible or rigid substrate, a magnetic layer, preferably formed from a Co or Co alloy film, and an underlayer comprised of a material having a Ga3Pt5 structure disposed between the substrate and the magnetic layer. As used herein, Ga3Pt5 structure means a material having a crystal symmetry like that of Ga3Pt5. The Ga3Pt5 structure has orthorhombic symmetry. Materials having a Ga3Pt5 structure include Ga3Pt5, Mn3Pd5, xcex4Ga3Ni5, InPt2, and Ni5Al3. Ni5Al3, the most studied of the Ga3Pt5 materials, is a crystallographic derivative of the face-centered cubic (xe2x80x9cFCCxe2x80x9d) structure. The underlayer may be formed in multiple layers wherein each layer is a different one of the foregoing FCC derivative materials, or wherein the layers alternate between a different one of the foregoing FCC derivative materials and a body-centered cubic (xe2x80x9cBCCxe2x80x9d) derivative material. The Co or Co alloy magnetic layer has a HCP structure deposited with its magnetic easy axis substantially parallel to the plane of the magnetic layer.
An overlayer that, in turn, may be covered by an overcoat may cover the magnetic layer. An organic lubricant is preferably added over the overcoat.
The recording medium may also include a first intermediate layer interposed between the magnetic layer and the underlayer. The first intermediate layer may be used to promote epitaxial crystalline growth of the magnetic layer. The first intermediate layer, if used, can consist of Cr, a Cr alloy, or a material having a BCC derivative crystalline structure, such as a material having a B2, DO3, or L21 crystalline structure. Cr has a BCC crystalline structure. A derivative structure of a basic structure is one in which one or more symmetry elements of the basic, or xe2x80x9cparentxe2x80x9d, structure (translational or orientational) is (are) suppressed. The basic periodicity and position of the atoms remains the same but the specific atomic occupancies change. BCC structures have many derivative structures, including but not limited to B2, DO3, or L21. The BCC structure has two atoms in its unit cell. The occupancy of the atom at (000) and that at (xc2xd, xc2xd, xc2xd) is the same. The same can be seen to be true for the other examples of derivative structures. The degree of atomic order increases:for each crystal structure in the sequence of crystal structures from BCC to B2 to DO3 to L21. Examples of suitable materials for the first intermediate layer include NiAl, AlCo, FeAl, FeTi, CoFe, CoTi, CoHf, CoZr, NiTi, CuBe, CuZn, AlMn, AlRe, AgMg, Mn3Si and Al2FeMn2. When NiAl is used as the material of the first intermediate layer, a strong NiAl (112) crystalline texture may be induced, thereby promoting an improved Co (1010) texture in the magnetic layer.
The recording medium may also include a second intermediate layer disposed between the first intermediate layer and the underlayer. The second intermediate layer may also be used to promote epitaxial crystalline growth of the magnetic layer. The second intermediate layer, infused, preferably consists of a material having a BCC derivative structure. When the material of the second intermediate layer consists of NiAl and the material of the first intermediate layer consists of Cr or a Cr alloy, a particularly strong Co (1010) texture may be induced on the magnetic layer by inducing a stronger NiAl (112) texture.
Yet a third intermediate layer may be placed in contact with the magnetic layer and the first intermediate layer. This layer may preferably consist of a non-magnetic HCP crystal structure such as a Coxe2x80x94Cr alloy, and is especially advantageous for providing a transition in lattice constant values from the BCC to the magnetic layer when the magnetic layer contains the larger atomic sized Pt element.
The novel underlayers of the present invention are FCC derivative structures. Their xe2x80x9cnaturalxe2x80x9d texture is the one with their close packed planes parallel to the plane of the film surface. These planes are the (221) planes of the Ga3Pt5 structure. These planes yield a very good match with the NiAl (112) on Cr (112) planes. Therefore, these new underlayers yield stronger (112) textures of the intermediate layers and, as a result, of the magnetic layers.
The novel underlayer of the recording medium of the present invention may be produced by a variety of methods. First, a bias voltage of about xe2x88x92300V may be applied to the substrate while a material with a B2 phase is deposited on the substrate. The bias voltage may be used to form the phases with the novel Ga3Pt5 structured underlayer of the present invention because of preferential sputtering of, for example, Ni from the NiAl target. The Co or Co alloy magnetic layer may then be grown on the underlayer. Second, and more preferably, the novel underlayer having a Ga3Pt5 structure may be deposited directly on the substrate from at least one target material or more, such as a target material having the 3:5 composition. The Co or Co alloy magnetic layer may then be grown on the underlayer, with or without the use of intermediate layer(s).
The invention also includes the target material for use in a sputter deposition process wherein the target material is sputtered onto a substrate to form the underlayer for the magnetic recording media of the present invention. The target material is made of a material that will form the Ga3Pt5 structure when deposited, and is preferably NiAl, wherein the atomic percentage of Ni is in the range of about 62 to 69% and the atomic percentage of Al is within the range of about 31 to 38%.